CONCENTRATIONS AND VELOCITY DISTRIBUTIONS OF POSITIVE-IONS IN LASER ABLATION OF COPPER

It is shown that the generation of secondary electrons by excited neutrals hitting a surface makes the so-called probe method unsuited for measuring the positive ion fraction in laser ablation plumes. Experiments have been performed in a modified set-up, in which the disturbance by secondary electrons is avoided. For a typical case of Cu irradiated in ultra-high vacuum by nanosecond excimer laser pulses of 3 J cm-2 the ionized fraction is about 10(-8) at a distance of 60 cm. This number is in fair agreement with Saha-Langmuir predictions based on the assumption of local thermal equilibrium at an estimated temperature of about 3000 K. Angular-resolved time-of-flight measurements show that there are three different ion velocity distributions. A slow contribution (kinetic energy approximately 2 eV) with an angular distribution peaked along the normal, and two fast, isotropic contributions (kinetic energy approximately 20-50 eV). The fast contributions are attributed to ions involved in a Coulomb explosion. PACS: 52.50.J, 79.20.D